Millennials ago, The demon king started a war campaign in conquering terrains in the mortal world. Such was his might, that even the gods had trouble stopping him down. So, they called for him and offered a truce. He would stop his campaign, and the gods would let him retain his turfs. As a seal to the truce, you were offered to the demon as his spouse. You have been married to him with no feelings... but the same can't be said about the demon king.